Generally, the display panel of the liquid crystal display is produced by applying a sealing material or sealant in the form of a frame to a glass substrate, bonding another glass substrate to the sealed glass plate from above and filling liquid crystal into a space between the glass substrates.
The conventional sealant application apparatus includes a head section having a nozzle mounted thereon, and a work table on which a glass substrate (work) is to be placed. By moving the head section and the work table relatively to each other into a predetermined position, a sealant can be applied linearly in a desired form.
In such a sealant application apparatus, it is necessary to keep a constant nozzle clearance (distance between the nozzle and work) for an acceptable shape of the applied sealant. In case the surface height of the work is not constant, the distance to the work is measured with the use of a non-contact distance sensor mounted on the head section, and the head section and work table are adjusted in height for a constant nozzle clearance before application of a sealant.
More particularly, a non-contact distance sensor (laser displacement sensor) is mounted on the head section with the nozzle, laser light is emitted from the sensor to the work, reflected light from the work is detected by the sensor to measure the distance L from the non-contact distance sensor to the work and the head section and work table are moved until the distance L becomes constant, as shown in FIG. 11 for example.
In the above sealant application apparatus, when no sufficient amount of sealant remains any more in the reservoir, the reservoir itself is replaced or it is replaced along with the nozzle; and if the nozzle is clogged, it is replaced or a new nozzle is installed. In such a case, it is possible that the existing and new nozzles are different in shape from each other, fixing tools used for the existing and new nozzles are mechanically different from each other or the new nozzle is installed with accuracy different from that with which the existing nozzle was installed (which is called “case error”). Such a case error will possibly cause the nozzle and non-contact distance sensor to be misaligned with each other in the direction of the nozzle clearance (Z-axial direction).
Even if the nozzle clearance is adjusted for the distance measured by the non-contact distance sensor to be the same as that which was before the nozzle replacement, the inaccurate alignment between the nozzle and non-contact distance sensor makes it impossible to reproduce precisely the nozzle clearance which was before the nozzle replacement. Thus, since the sealant cannot be applied in desired form, the nozzle clearance has to be adjusted taking in consideration the Z-axial displacement between the discharge port of the nozzle and non-contact distance sensor each time the nozzle is replaced or a new one is installed.
To solve the above problem, there has been proposed a nozzle clearance adjusting method in which a measuring jig is placed on a work table, the distance over which the nozzle has to be moved until it touches the measuring jig is measured by an optical displacement gage (non-contact distance sensor) and the nozzle and optical displacement gage are aligned between them based on the measured distance to adjust the nozzle clearance (as disclosed in the Japanese Unexamined Patent Application No. H05-15819).
In the method disclosed in the above Japanese Unexamined Patent Application No. H05-15819, however, even if the distance measurement is made by the non-contact distance sensor with the discharge port of the nozzle put into touch with the measuring jig in case the surface of the measuring jig has an Z-directional error such as undulation, a position where the nozzle is put in touch with the measuring jig and a position where the distance is measured by the non-contact distance sensor are misaligned with each other. So, accurate information on the Z-axial relative position between the nozzle and non-contact distance sensor cannot be provided, and thus the nozzle clearance cannot accurately be adjusted.
Also, since the position where the nozzle is in touch with the measuring jig and the position where the measurement by the non-contact distance sensor is made are separated from each other, a slight inclination of the measuring jig will result in a large Z-axial misalignment and tremendous amounts of labor and cost are required for production of a high-precision measuring jig.
Further, in the Japanese Unexamined Patent Application No. H05-15819, when the nozzle is put into touch with the measuring jig, abutment of the nozzle causes the surface of the measuring jig to incur strain. While such strain is taking place, the distance measurement is made by the non-contact distance sensor. So, it is not possible to accurately measure the Z-axial alignment between the nozzle and measuring jig, and thus the nozzle clearance cannot be adjusted accurately.
Also, when the nozzle is put into touch with the measuring jig as above, it will possibly be damaged at the front end thereof, so that no acceptable sealant application can be assured.
It is an object of the present invention to overcome the above-mentioned drawbacks of the conventional art by providing a method of adjusting the nozzle clearance in a liquid application apparatus, in which the nozzle and non-contact distance sensor can accurately be positioned relatively to each other in the direction of the nozzle clearance and the nozzle clearance can accurately be adjusted under less influence of undulation or inclination of the work, and a liquid application apparatus adapted so that the nozzle clearance is so adjustable.